Question

I am just reading a great tutorial about threads and have a problem with locks. I need some tip/advice that will point me in right direction. I’d like to understand why the output isn’t ordered as i expect. The code shows my simple example.

  class Program {
    class A {
      public object obj = new object();
      public int i;
    }
    class B {
      public object obj = new object();
      public int j;
    }
    static void Main() {
      Console.Write("Thread1: ");
      A a = new A();
      for (a.i = 0; a.i < 9; a.i++) {
        lock (a) {
          new Thread(() => { Console.Write(a.i); }).Start();
        }
      }
      Thread.Sleep(500);
      Console.Write("\nThread2: ");
      B b = new B();
      for (b.j = 0; b.j < 9; b.j++) {
        new Thread(() => { lock (b) { Console.Write(b.j); } }).Start();
      }
      Console.ReadLine();
    }
  }

Example output:
Thread1: 222456799
Thread2: 233357889

Link to the tutorial:
http://www.albahari.com/threading/

Answer 1

You are only locking while you create the thread, or (in the second case), access the value. Locks must be used by all threads, otherwise they do nothing. It is the act of trying to acquire the lock that blocks. Even if you did lock in both threads, that wouldn’t help you marry each thread to the value of a.i (etc) at a particular point in time (that no longer exists).

Equally, threads work at their own pace; you cannot guarantee order unless you have a single worker and queue; or you implement your own re-ordering.

it will run at its own pace, and since you are capturing the variable a, it is entirely likely that the field a.i has changed by the time the thread gets as far as Console.Write. Instead, you should capture the value, by making a copy:

  A a = new A();
  for (a.i = 0; a.i < 9; a.i++) {
    var tmp = a.i;
    new Thread(() => { Console.Write(tmp); }).Start();
  }

(or probably remove a completely)

  for (int i = 0; i < 9; i++) {
    var tmp = i;
    new Thread(() => { Console.Write(tmp); }).Start();
  }